The coccidian Toxoplasma gondii is an intracellular parasite that can infect all mammalian cells. While infections of immunocompetent individuals are usually asymptomatic, toxoplasmosis in immunocompromised individuals can cause serious illness or death. While effective therapies exist for the treatment of toxoplasmosis, they are not without adverse effects, thus establishing a rationale for vaccine therapy. Because immunization with nucleic acids can induce both CD4+ and CD8+ mediated immune responses, and because both types of responses are important in conferring immunity to T. gondii infections, immunization with DNA has the potential to induce protective immunity. To assess the feasibility of achieving immunity to T. gondii, we performed vaccinations of BALB/c mice using a cDNA construct that contained the coding sequence for the mature peptide of the P30 surface antigen. Mice of five different H-2 haplotypes elicited antibody titers in excess of 103 when immunized by intramuscular injection. Cultured splenocytes from immunized mice produced IFNg when stimulated with P30, indicating that a Th1 response had developed. Upon oral challenge with a normally lethal dose of T. gondii tissue cysts, nine of ten immunized mice survived while ten of ten control mice died. When animals were challenged with a lower, nonlethal dose, no differences were observed in the tissue burden of brain cysts. These results indicate that polynucleotide vaccination with a P30 construct affords protection from acute toxoplasmosis, but is not sufficient to prevent the establishment of tissue cysts.